[PATCH] Improve performance of lazy linking in llvm-link

[James Molloy]

Hi,

The attached patch improves the runtime performance of LinkModules. When
linking even a small module against a large (for example a library)
module with many demand-linked symbols (available_externally linkage),
the linker ends up taking much longer than it should.

For every function in the library module (whether it will be linked or
not), it creates an equivalent function declaration in the source
module. Then, at the end of the link, it checks the usage count of the
declaration - if it's non-zero it needs to be linked. If it's zero, it
doesn't need to be linked and can be removed.

This is very neat in terms of the code structure, but the creation and
erasing of these function declarations does take quite a bit of time
with a large library module.

The attached patch changes the algorithm to be slightly less "neat" in
coding style but much more performant.

Please review!

Cheers,

James

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